Hyperbranched polyesters belong to the family of dendrimers bearing large number of functional groups on their periphery. Despite their high polydispersities and low degree of branching, their chemical performances are quite similar to their dendrimer analogues.
Topological specifics of hyperbranched polymers like high level of solubility and compatibility, low viscosity of solutions, resistance to aggregate even in concentrated solutions, ability to function as nanocontainers for substances sorbed inside, weak dependence of hydrodynamic volume on molecular weight and lump of free ends of chains with functional groups on their periphery open a new window for their diverse industrial use.
Hyperbranched polymers find wide applicability as cosmetics, food additives, surfactants, lubricants, plasticizers, drug delivery agents, azeotropic phase separators, in orthopaedic and ophthalmic fields, building blocks for preparing poly-addition or poly-condensation polymers, additives in paints, coverings, adhesive promoters, sealants, casting elastomers, as rheology modifiers and as surface or interface modifiers. Discovery of unique properties of hyperbranched polyesters has expedited a rapid development in this area of materials. Hyperbranched polyesters derived from renewables broaden their applicability as novel, environmental-friendly materials for various applications.
Polyesters are conventionally synthesized by polycondensation of di- or polycarboxylic acids with polyols. Industrially, aromatic polyesters, i.e., polyesters from aromatic dicarboxylic acids such as phthalic acid or terephthalic acid and alcohols such as 1,2-ethane diol, 1,2- or 1,3-propane diol or 1,4-butane diol, are more significant. The special applications of aliphatic, biodegradable polyesters derived from parent molecules like succinic acid, glutaric acid or adipic acid and alcohols such as 1,2-ethane diol, 1,2- or 1,3-propane diol, 1,3- or 1,4-butane diol, 1,5-pentane diol or 1,6-hexane diol are making these polymers more attractive in recent years.
U.S. Pat. No. 6,441,126 and US 20100048813 describes a process for producing cross-linked, branched, aliphatic polyesters for gum base from glycerol and adipic acid at 150-200° C. without any catalyst under solvent-free conditions.
EP 0799279 discloses the synthesis from an ethoxylated pentaerythritol and 2,2-dimethylol propanoic acid using conventional homogenous mineral acid catalyst, H2SO4, at 140° C. Similarly several prior arts are available that disclose synthesis of polyesters from different reactants employing different catalysts, in solvent-free conditions or in the presence of solvent. Enzymatic and microbial catalysed processes also form part of prior art in this area. The results of such prior art processes are polyesters that do not possess the characteristics of low viscosity at a high degree of branching and with controlled gelation.
There are certain drawbacks with the prior art processes which include that the polyesters produced have high viscosity and lower degree of branching. The final product is always a gel. Controlled gelation at high conversions is an issue. Tuning the topological characteristics of hyperbranched polyesters is desirable to focus their application in specialized fields. Recovery of homogeneous catalysts and their reusability is also an issue with the mineral acid and organometal catalysts. Further, the organometal catalysts e.g., dibutyltin, butylstanoic acid etc are air and moisture sensitive. As water is formed as a by-product in the polyesterification reaction, the organometal catalysts soon get deactivated. Corrosion of reactors and pipe lining is a problem with mineral acid catalysts. Although the reaction occurs at mild temperature while using enzyme catalysts, long reaction times of nearly 24 h are needed. A solid catalyst-based process has several engineering, environmental and economic advantages. The solid catalyst can be easily separated and reused in subsequent recycles. It is therefore desirable to develop an efficient solid catalyst-based process for producing hyperbranched polyesters of low-viscosity and high degree of branching with controlled gelation to achieve polyesters for speciality applications.
Double-metal cyanides were used as catalysts for the preparation of polyether polyols (WO 2009055436; U.S. Pat. No. 6,624,286; Chen et al., J Polym Sci A Polym Chem., Year 2004, Vol. 42, pp. 6519), alternative copolymerization of epoxides and CO2 (U.S. Pat. Nos. 6,359,101, 6,953,765 and 6,852,663) and biodiesel and biolubricants manufacture by transesterification/esterification of vegetable oils or animal fat (monohydric carboxylic acid) with monohydric alcohols (U.S. Pat. Nos. 7,754,643, 7,482,480 and 7,842,653; EP 1 733 788; Sreeprasanth et al., Appl. Catal. A: Gen., Year 2006, Vol. 314, pp. 148). However, their application for the preparation of hyperbranched polyesters is not disclosed so far.
It is surprising that double metal cyanide catalysts with cubic crystallite structure, micro-mesoporous architexture, strong Lewis acid centers and surface hydrophobicity exhibit high catalytic activity to produce hyperbranched polyesters especially those with low-viscosity, high degree of branching and controlled gelation.
Double metal cyanide catalysts of different crystallite structures (monoclinic) and amorphous nature have been used for the preparation of polyesters and other hybrid polymers as described in the following prior arts.
DE 10 2008 004 343 A1 discloses a process for preparing polyester alcohol useful to prepare polyurethane foams and thermoplastic polyurethane elastomers, comprising catalytic conversion of at least a difunctional carboxylic acid with at least a difunctional alcohol, where at least a part of the reaction is carried out in the presence of a polymetal cyanide or a double metal cyanide catalyst usually in an amount up to 1% by weight of the reaction mixture. The polycondensation reaction can be performed both in presence and in absence of a solvent at temperature 160-280° C. This disclosure is specific for conversion of dicarboxylic compounds with dihydroxy compounds. The product is a linear polyester whereas the instant invention describes a hyperbranched polyester with low-viscosity and high degree of branching at controlled gelation.
WO 2011/137011 A1 tells a process for preparing a hybrid polyester-polyether polyols comprising reacting a carboxyl group-containing component and an epoxide, optionally in the presence of one or more double metal cyanide catalyst, a superacid catalyst, a metal salt of a superacid catalyst and/or a tertiary amine catalyst, under conditions such that a hybrid polyester-polyether polyol is formed. Further, the said reaction is carried out in presence of solvent such as toluene. The process results in products having narrow polydispersity, a low acid number and unsaturation and reduced byproducts formation, particularly when the metal cyande catalyst is employed. The product is a hybrid polyester-polyether-polyol, different from that of the present invention. US 7842653 describes a process for preparing lubricants wherein the said process comprises contacting a monocarboyxlic vegetable oil or fat with a monohydric C6-C8 alcohol in the presence of a double-metal cyanide catalyst wherein the vegetable oil/fat to alcohol molar ratio being 1:6, reaction temperature being in the range 150-200° C. and reaction time being in the range 3-6 hrs. The product is a monoester.
WO 2011/075343 A1 discloses a process to convert a secondary hydroxyl-capped polyol to a primary hydroxyl-capped polyol comprises reacting a polyether polyol, a polyester polyol, polyether-ester polyol or a polyether-polyester polyol with a cyclic anhydride of a polycarboxylic acid to form a half acid ester, followed by the reacting the half acid ester with ethylene oxide, to form a polyester polyol or a polyether-polyester polyol. Both steps are carried out in the presence of an amine catalyst and a double metal cyande catalyst. The latter catalyst useful for polymerization of epoxides generally may include an organic complex agent such a diglyme.
CN101570595 discloses a terpolymer containing polyester chain links and polycarbonate chain links and a synthetic method thereof, which comprises the mixing of zinc-cobalt double metal cyanide complex catalyst, cyclohexene oxide and dicarboxylic anhydride in a solvent.
The double metal cyanide catalyst used in the present invention is unique and thereby shows efficient catalytic activity for the preparation of hyperbranched polyesters. The unusual surface hydrophobicity of the catalyst facilitates the adsorption of polycarboxylic acids and polyols but not the water formed as by-product in the esterification reaction and micro-mesoporous architecture controls the gelation and degree of branching.